Shaft current is a common and unavoidable problem for high-voltage motors and variable-frequency motors. Shaft current can cause great damage to the bearing system of the motor. For this reason, many motor manufacturers will use insulating bearing systems or bypass measures to avoid shaft current problems.
Shaft current is generated because a magnetic flux that changes with time passes through the loop composed of the motor shaft, bearings and bearing chamber, inducing shaft voltage on the shaft and generating current when the loop is turned on; it is a low-voltage, high-current physical phenomenon that causes great damage to the motor bearing system and will destroy the bearing due to electrocorrosion in a short time.
The motor core punching is a fan-shaped piece with a slot positioned with the base on the punching; the split core of a large motor and the eccentricity of the rotor are key factors in the generation of shaft current. Therefore, shaft current has become the main problem of large motors.
In order to avoid the shaft current problem, necessary measures should be taken in the selection and design of parts and components to theoretically eliminate the factors that generate shaft current. The number of joints S on the circumference is controlled and adjusted by the relationship between S and the greatest common divisor t of the number of motor pole pairs.
When S/t is an even number, there is no condition for generating shaft voltage, and naturally there will be no shaft current; when S/t is an odd number, it is very likely that shaft voltage will be generated, and shaft current will be generated. Even if this type of motor is an industrial frequency motor, there will be shaft current problems. Therefore, for large motors, measures to avoid shaft current are generally taken.
In addition, variable frequency motors are also one of the reasons for generating shaft current because of the high-order harmonics of variable frequency power supply. No matter how powerful the variable frequency motor is, there may be shaft current. Therefore, many small-power variable frequency motors will use insulated bearings, while most high-power motors will use insulated end covers, or take insulation measures on the shaft bearing position; some manufacturers, in order to ensure the commonality of variable frequency motors and ordinary industrial frequency motor components, will take bypass measures at the bearing cover position.